Field-sequential color display is known as a typical method for representing color images without using color filters. Devices of this type include light sources of three colors: red, green and blue; and a monochromatic display element. These color display devices produce color images by depicting monochromatic images with the monochromatic display element and synchronously switching the three color light sources on and off at a high speed. The monochromatic display element can be either reflection-type ones or transmission-type ones.
Color display devices of this type using no color filters (of a field sequential system) have various applications. Such examples are applications to liquid crystal displays and other monochromatic display elements. Further examples are applications to projectors. In case of projectors, a resolution equivalent to the resolution of a three-panel projector not using such a color display device can be achieved with a single-panel projector using the color display device, and the projectors can be simplified in optical system. Because of these and other advantages, applications to projectors have been marketed. A still further example is application to head mount displays that users can wear on heads to position their display portions in front of the users' eyes. In this application, because of their merit, i.e. compact and high-resolution color representation, some color display devices of the field sequential system have been proposed heretofore (Patent Documents 1 and 2). Furthermore, prototypes of large-scale, direct-view-type display devices have been demonstrated as well.
For easier understanding of the description hereinbelow about color display devices using the field sequential system, the following definitions are given. First of all, a minimum cycle in which three color light sources of red, green and blue sequentially turn on and off to display any given color is herein called a “display frame”, and its length of time is called “one display frame period”. A minimum cycle for displaying any one of the three colors is herein called a “color field”, and its length of time is called “one color field period”.
In existing field sequential color display methods, red, green and blue light sources are sequentially turned on and off once for each color as shown in FIG. 1, and the display frame comprising red→green→blue, for example, is repeated.
FIG. 2 is a block diagram that illustrates a drive circuit for an existing field sequential display device. FIG. 3 is a timing chart showing timings in the existing field sequential display device.
With reference to FIGS. 2 and 3, timings of image signals in the conventional field sequential display devices are explained. The drive circuit of FIG. 2 for the existing field sequential display device includes two frame memories that are a first frame memory and a second frame memory. These frame memories are controlled by a memory arbitration circuit to each write and read an image of one frame independently from each other.
With reference to FIG. 3, during a frame period in which “Image 1” is input as an image signal, data of “Image 1” is written in the first frame memory. Simultaneously in the same frame period, data of “Image 0”, which was written in the second frame memory in the preceding frame period, is read out in the second frame memory in the order of the red color field, green color field and blue color field. After the data of each color image is written, the light source of each color is turned on and off, thereby representing a color image.
In the next frame period in which “Image 2” is input, data of “Image 2” is written in the second frame memory. Simultaneously in the same frame period, data of “Image 1”, which was written in the first frame memory in the preceding frame period, is read out in the first frame memory in the order of the red color field, green color field and blue color field. After the data of each color image is written, the light source of each color is turned on and off, thereby representing a color image.
One of known problems with existing color display devices of the field sequential system is the issue of flickering. Flickering, however, may occur in other systems other than the field sequential system as well. It has been generally acknowledged that the on-and-off cycle (period) of red, green and blue light sources must be shorter than 1/60 second (Patent Document 3) for observers to be insensitive to flickering. Actually, however, the higher the luminance of color display devices, the higher the frequency level for human eyes to become insensitive to flickering. With this knowledge, the present Inventor already proposed a color display method for observers to be less sensitive to flickering (Patent Document 4).
As shown in FIG. 4 that shows a feature of this prior method of the present Inventor himself, this method proposes to compose one display frame of four color fields to turn the green light source on and off twice in each display frame in the order of, for example, red→green→blue→green and repeat this cycle.
Frequency at which human eyes become insensitive to flickering varies with color. Frequency at which most observers do not sense flickering of green color is twice that of red color or blue color. Therefore, the method shown in FIG. 4 in which the on-off frequency of green color is doubled relative to the frequency of display frames can keep observers insensitive to flickering even at a lower frequency of display frames. In the method of FIG. 4, each red color field and each blue color field intervene respectively between two green color fields, and each green color field results in being interposed between color fields of a composite color of red and blue in average. This is advantageous for a better uniformity of colors on a display screen even when the monochromatic display element used has a slow response speed. This property is especially useful with field sequential liquid crystal panels using nematic liquid crystal that has a relatively slow speed of behavior.
There is the problem of color breakup peculiar to color display devices of the field sequential system. It is generally said that color breakup occurs when motion pictures are displayed by field sequential color display devices. More specifically, with reference to FIG. 5, when a white square image moves in a black background, false colors are perceived along the boundaries intersecting the moving direction of the white square image.
“Color breakup in motion pictures” derives from the timings of storing general image signals in frame memories and retrieving them for respective color fields that are peculiar to the field sequential system. FIG. 6 illustrates how such color breakup appears in the conventional field sequential system.
Explained below are causes of color breakup with existing field sequential display devices with reference to FIGS. 3, 5 and 6.
Taking an existing field sequential display device with which the white square image is being displayed as moving at a uniform speed in the black background as shown in FIG. 5, FIG. 6 shows at (a) the motion of the white square image along the horizontal axis and its time along the vertical axis. As shown in FIG. 3, data of images of an identical position are outputted to the liquid crystal panel in one frame period and in one display frame period. Observer's eye tracking, however, moves at a constant speed following the motion of the image. This is shown by an inclined line at (a) of FIG. 6. FIG. 6 also shows at (b) relative positions of the image being displayed when assuming that the observer's eyes stay at a constant position. The image being displayed is recognized by the observer's eyes as an integrated, synthesized image. In the case where the white square image is moving at a uniform speed, positions displayed by respective colors are constant respectively as shown at (b) of FIG. 6, and this causes the observer to perceive color breakup shown by references III and IV at (b) of FIG. 6.
Additionally, in existing methods in which one frame consists of three color fields, if the display element used has a slow response speed, it cannot complete its full response before the light source of the next color field turns on, and the color of the next color field is undesirably mixed accordingly. As a result, the actually displayed color of the red color field appears yellowish with mixture of green. Similarly, the actually displayed color of the green color field becomes cyan-tinged, and the actually displayed color of the blue color field becomes magenta-tinged.
Therefore, display devices using liquid crystal as its display element, have the drawback that correct color representation is not possible especially in a low temperature range where the response speed of the liquid crystal becomes slower and hues of the colors are liable to rotate on the chromaticity diagram.
Existing display methods have a still further problem that color ununiformity occurs between the top and the bottom of the display screen. This is because, due to a time lag between the timing of writing data at the top of the screen and the timing of writing data at the bottom of the screen during writing of data while scanning the screen, degrees of color ununiformity differ from the top to the bottom of the display screen.
Color breakup of motion pictures can be reduced by an appropriate control of timings of frame memories. Heretofore, however, color breakup could not be removed completely even by such a timing control. This is because there is the problem of “saccadic color breakup” in addition to “color breakup of motion pictures”.
“Saccadic color breakup” is a phenomenon in which a false color of a high saturation, not contained in the image displayed, is perceived randomly, momentarily. This occurs when eyes are moved swiftly because it causes a large movement of the viewing point in one frame period, which invites a large displacement of the image of red, green and blue.
It is a characteristic nature of saccadic color breakup that it occurs not only with motion pictures but also with still pictures, and it prominently occurs at boundaries between white and black of images exhibiting a high contrast. Increasing the frame frequency is a measure for removing this problem. Heretofore, however, this saccadic color breakup could not be removed even in projectors using the field sequential system that were doubled in frequency as compared with the normal frequency. This problem has been an obstacle against promulgation of color display devices using the field sequential system. Furthermore, since the approach by increasing the frame frequency results in increasing the power consumption of the display device, it has been difficult to increase the frame frequency in display devices using certain kinds of monochromatic display elements because of limited speeds of behavior of such display elements.    Patent Document 1: Japanese Patent Laid-open Publication No. 2010-32997    Patent Document 2: Japanese Patent Laid-open Publication No. 2009-251319    Patent Document 3: Japanese Patent Laid-open Publication No. 2007-206698    Patent Document 4: International Publication No. WO 01/095303 (International Application No. PCT/JP01/04813)